Near‐field postseismic deformation associated with the 1992 Landers and 1999 Hector Mine, California, earthquakes

Abstract

After the Landers earthquake (Mw = 7.3, 1992.489) a linear array of 10 monuments extending about 30 km N50°E on either side of the earthquake rupture plus a nearby off‐trend reference monument were surveyed frequently by GPS until 2003.2. The array also spans the rupture of the subsequent Hector Mine earthquake (Mw = 7.1, 1999.792). The pre‐Landers velocities of monuments in the array relative to interior North America were estimated from earlier trilateration and very long baseline interferometry measurements. Except at the reference monument, the post‐Landers velocities of the individual monuments in the array relaxed to their preseismic values within 4 years. Following the Hector Mine earthquake the velocities of the monuments relaxed to steady rates within 1 year. Those steady rates for the east components are about equal to the pre‐Landers rates as is the steady rate for the north component of the one monument east of the Hector Mine rupture. However, the steady rates for the north components of the 10 monuments west of the rupture are systematically ∼10 mm yr−1 larger than the pre‐Landers rates. The relaxation to a steady rate is approximately exponential with decay times of 0.50 ± 0.10 year following the Landers earthquake and 0.32 ± 0.18 year following the Hector Mine earthquake. The postearthquake motions of the Landers array following the Landers earthquake are not well approximated by the viscoelastic‐coupling model of Pollitz et al. [2000]. A similar viscoelastic‐coupling model [Pollitz et al., 2001] is more successful in representing the deformation after the Hector Mine earthquake.